Secondary recovery using carbon dixoide and an inert gas

ABSTRACT

Recovery of oil from an oil-bearing reservoir is effected by the injection of a mixture of carbon dioxide and an inert gas in a critical ratio whereby there is formed a miscible transition zone between the mixture and the reservoir oil.

FIR /912 United States Patent 11 1 1111 3,811,501

Burnett et a]. 1 May 21, 1974 1 1 SECONDARY RECOVERY USING CARBON 3,256,933 6/1966 Murphree et :11. 166/273 x Q E AND AN INERT GAS 3,623,552 11/1971 Vairogs.. 166/274 3,262,498 7/1966 Connaliy, Jr. et a1. 166/273 [75] In ento s: av d Burn'ett, Houston; b t 3,687,198 8/1972 Hearn et a1. 166/274 B. Alston, Missouri; Frank H. Lim, 3,620,304 1 1/1971 Hearn et a]. Houston, all of Tex. 3,661,208 5/1972 Scott et al 3,249,157 5/1966 Brigham et a1.-....;. Asslgfleei Texaco -9 New York, 2,875,830 3/1959 Martin 166/273 x 122 P1166: Ju1y 27, 1972 P E S J N d 1 rimary xaminertep en ovosa [21] Appl' 275587 Attorney, Agent, or Firm-T. H. Whaiey; C. G. Reis [52 US. Cl 166/252, 166/273, 166/274 57 ABSTRACT [51] Int. Cl B21b 43/22 1 58 Field 61 Search 166/252, 273, 274 Recovery of 011 from Oflbearmgreservmr IS fected by the injection of a mixture of carbon dioxide- [56] References Cited and an inert gas in a critical ratio-whereby there is UNITED STATES PATENTS formed a miscible transition zone between the mixture and the reserv01r 011. 2,623,596 12/1952 Whorton et al 166/274 3,084,743 4/1963 West et al 166/273 9 Claims, 1 Drawing Figure HYPOTHETICAL CARBON DlOXlDE-HYDROCARBON SYSTEM AT PRESSURE P AND TEMPERATURE T INTERMEDIATES Y VV /\/\W VX 'T E LINE Amm.

AW fVVMVVVV AAA/W ATENTEnmzr i974 (181 1,501 P HYPOTHETICAL CARBON Dl lDE-HYDROCARBON SYSTEM AT PRESSURE P TEMPERATURE T INTERMYEDIATES SECONDARY RECOVERY USING CARBON DIXOIDE AND AN INERT GAS FIELD OF THE INVENTION DESCRIPTION OF THE PRIOR ART In the recovery of oil from an underground reservoir one method that has been suggested for improving oil recovery is that of miscibleflooding wherein a solvent is injected into the reservoir to push and wash out the oil of the reservoir. When solvents are employed which can mix completely with the oil, the term miscible flooding is applied to the process.

The process of miscible flooding is extremely effective in stripping and displacing the reservoir oil from the reservoir thru which the solvent flows. This effectiveness is derived from the fact that a two-phase system within the reservoir and between the solvent and the reservoir is eliminated at the conditions of temperature and pressure of the reservoir, thereby eliminating the retentive forces of capillarity and interfacial tension which are significant factors in reducing the recovery efficiency of oil in conventional flooding operations where the displacing agent and the reservoir oil exist as two phases in the reservoir.

Miscible recoveries of oil are normally accomplished by displacement techniques wherebya fluid that is miscible with the reservoir oil is injected into a reservoir which serves .to displace the oil thru the reservoir and toward a production well from which the oil is produced. Normally, the fluids used are light hydrocarbons and mixtures thereof, such as paraffins in the C to C range and, in particular, liquid petroleum gas or LPG.

Carbon dioxide has been used as an oil recovery agent wherein recovery is improved by taking advantage of the solubility of the carbon dioxide in the oil, causing viscosity reduction and swelling of the oil, thereby leading to increased recovery. However, in this use, carbon dioxide is not a miscible type displacing agent since, the pressures have been much lower than the miscibility pressures for carbon dioxide and oil.

It is an object of the present invention to better utilize carbon dioxide as a recovery agent by means of a conditional miscible flood process, wherein conditional miscibility of the carbon dioxide in the reservoir oil occurs at the reservoir conditions of temperature and pressure.

It is a further object of this invention to utilize an inert gas with the carbon dioxide so as to decrease carbon dioxide requirements while retaining the benefits of miscibility, thereby enhancing theeconomics of the miscible flood. It is yet a further object of this invention to provide a technique for determining the critical ratio of carbon dioxide to inert gas to achieve miscibility with the reservoir oil.

SUMMARY OF THE INVENTION This invention relates to the recovery of oil from an oil-bearing reservoir wherein a slug of a mixture of carbon dioxide and an inert gas in critical ratio is injected into the reservoir in an amount sufficient to form a conditional miscible transition zone with the reservoir oil at the reservoir conditions of pressure and temperature, and thereafter injecting a driving agent to displace the reservoir oil toward a production well from which the oil is produced.

BRIEF DESCRIPTION OF THE FIGURE The FIGURE illustrates a three-component composition diagram for a carbon dioxide-hydrocarbon system.

DESCRIPTION OF THE INvENTIoN In its broadest aspect, the invention comprises intro- 0 ducing into the oil-bearing reservoir a slug of a mixture of carbon dioxide and an inert gas that is capable of forming a miscible transition zone with the reservoir oil, and thereafter injecting a driving fluid such as natural gas to displace the oil thru the reservoir to a produc- 5 tion well from which it is produced.

The invention resides in the fact that the reservoir is flooded under conditions at which conditional miscibility exists between the slug mixture and the reservoir oil. It is within the scope of the invention to provide a meansfor determining the critical ratio of the carbon dioxide to the inert gas.

Conditional miscibility within the meaning of this invention is to be distinguished from instant miscibility by the fact that miscibility in a conditional miscibility sense is achieved by a series of transition multiphase conditions wherein the injected mixture vaporizes intermediate components from the oil until a miscible transition zone is created in situ by a vaporizing gas drive.

According to the invention, such conditional miscibility may be achieved not only by the use of carbon dioxide but also by the use of carbon dioxide containing inert gas. The ratio of the inert gas to the carbon dioxide at which conditional miscibility may be attained for a particular reservoir crude and at particular reservoir conditions, is termed the critical ratio. At this ratio the mixture contains the maximum concentration of the inert gas that can be present for the practice of this invention, i.e., to have conditions of conditional miscibility. v

The critical ratio for the condition relating to a vaporizing gas drive as herein defined can be detennined by means of slim tube displacement tests, which tests are commonly used to establish conditions simulating an enriched gas drive. An enriched gas drive, as contrasted with a vaporizing gas drive, is defined as one in which absorption of the components of. the slug solvent by the reservoir oil occurs.

A further insight into the invention can be obtained by referring to the accompanying FIGURE. A hypothetical carbon dioxide, intermediates (C -C oil system may be considered on a pseudo threecomponent system as shown at a given pressure and temperature. The vertices of the triangle are represented by carbon dioxide, intermediates and oil. The curve M, C, N represents the phase envelope and the area under the phase envelope, l, is a two-phase region in which a liquid and a gas phase exist. The area above the phase envelope, lll, represents a single phase condition which is liquid. The area to the left of the phase envelope, 11 represents a single phase condition which is gaseous. The tie line whichis shown tangent to the phase envelope and thru the critical point, C, repre-' sents the limiting tie line for miscible conditions where the fluid in the reservoir such as A is sufficiently rich in intermediates so that the reservoir may be subjected to miscible drive. For a given solvent-oil system, there will be some minimum pressure above which a miscible drive occurs.

We have discovered that for any given pressure greater than this minimum pressure, there is also a solvent dilution factor whereby the carbon dioxide may be diluted with an inert gas and still have conditional miscibility in the reservoir.

By an inert gas we mean a gas with a solubility in reservoir oil less than carbon dioxide at the reservoir conditions of temperature and pressure. Examples of an inert gas are methane, natural gas, separator gas, flue gas, nitrogen and air. The maximum concentration of the inert gas that can be present with the carbon dioxide and yet have conditional miscibility may be determined by the slim tube displacement tests. By means of these tests, percent recovery of the in-place fluid is determined at solvent breakthru for a given composition of solvent mixture by varying the composition at the pressure of the reservoir. A breakpoint is determined in a curve of percent recovery vs. composition. This breakpoint is indicative of the inception of conditional miscible-type behavior. In conducting slim tube tests, a 40-foot long tube is filled with sand which is then saturated with the oil of interest. A displacing fluid, having a composition containing carbon dioxide and an inert gas, is injected at a given rate, and thereafter displacement of the oil is monitored by means of observing the effluent from the tube. Observation of the first appearance of a gas phase is noted in a high pressure sight glass and recovery is determined at the point in time of this appearance of two phases.

In a series of tests using a given reservoir oil, displacement studies were conducted at a pressure of 1500 psi and a temperature of 130 F. Under these conditions, and for this particular oil, conditional miscibility was obtained for a slug of 100 percent carbon dioxide and the percent recovery was approximately 96 percent. Additional tests in which the percent of inert gas, i.e., methane was increased, gave the following results:

Vol. Percent Percent Methane Recovery From the results plotted, approximately 90% recovery was realized when the percent composition of the methane in the slug was 7.7 percent. The basis for indication of conditional miscibility at a 90 percent recovery is found in prior art wherein it is stated that if recovcries are 90 percent or greater, miscibility is indicated in hydrocarbon systems. In this regard note Miscible Fluid Displacement-Prediction of Miscibility, A. L. Benhem, W. J. Kunzman, PETROLEUM REPRINT SERIES, No. 8, p. 123.

After having established a slug of a mixture of carbon dioxide and inert gas, and the transition zone between the reservoir oil and the slug, a driving fluid is then injected to drive the slug, the transition zone and the reservoir oil thru the reservoir towards a production well from which the oil can be produced. The driving fluid may be any relatively inexpensive fluid, including gas such as air, nitrogen, combustion or flue gas, separator gas, natural gas, or mixtures thereof. The drive fluid may also be water or brine and may contain additives such as a surfactant, to maintain efficient displacement by the driving fluid.

The driving agent is injected in amounts sufficient to displace the mixture of the slug thru the reservoir and is injected at a rate so that the preferred rate of movement thru the reservoir is from about 0.03 to about 10.0 feet per day.

It is within the scope of the invention to apply the process either as a horizontal displacement technique wherein the slug is established to form a circumferential ring expanding from the injection well as the process is continued, or it can be used in a vertical displacement wherein a blanket of the slug is established prior to the inejction of the driving fluid and which moves the blanket vertically thru the reservoir.

In summary, in accordance with the practice of this invention, a conditional miscible flood is carried out in the following manner. There is introduced into the reservoir a slug of a mixture of carbon dioxide and inert gas that is capable of forming with the reservoir oil, at the temperature and pressure thereof, a zone of conditional miscibility. The composition of the mixture, or the critical ratio of the components may be determined by means of slim tube displacement tests. After an amount sufficient to establish a slug has been injected, there is introduced into the formation a driving fluid such as gas or water. The injection of the driving fluid is continued so as to move the fluids of the reservoir thru the reservoir toward a production well from which the reservoir oil and the solvent can be produced. By operating in the above-indicated manner, a substantially complete displacement of the reservoir oil is realized.

We claim:

1. A process for recovering oil from an oil-bearing reservoir traversed by at least one injection well and one production well, comprising the steps of:

a. determining the critical ratio of inert gas to carbon dioxide to achieve conditional miscibility of said inert gas and carbon dioxide with the reservoir oil at the pressure and temperature of said reservoir,

b. injecting into said reservoir via said injection well a slug comprising said inert gas and said carbon dioxide in said critical ratio, said slug being injected in amounts sufficient to establish a transition zone of said slugwith said reservoir oil,

0. injecting into said reservoir a driving agent and driving said oil thru said reservoir toward said production well,

d. producing said oil via said production well.

2. The process of claim 1 wherein said inert gas is se lected from the group consisting of methane, natural gas, separator gas, flue gas, nitrogen, air and mixtures thereof.

3. The process of claim 2 wherein said inert gas is natural gas.

4. The process of claim 1 wherein said driving agent is selected from the group consisting of air, nitrogen, combustion gas, separator gas, natural gas, methane, water and mixtures thereof.

5. A process for recovering oil from an oil-bearing reservoir traversed by at least one injection well and one production well, comprising the steps of:

a. injecting into said reservoir via said injection well a slug comprising carbon dioxide and an inert gas, said slug being capable of forming a conditionally miscible transition zone with said oil at the pressure and temperature of said reservoir, said slug being injected in amounts sufficient to establish said transition zone,

b. injecting into said reservoir via said injection well a driving fluid and driving said slug and said oil thru said reservoir towards said production well,

0. producing said oil via said production well.

6. The process of claim 5 wherein step (a) is preceded by injection of a fluid to repressure said reservoir to the minimum conditional miscibility pressure at which said slug is miscible with said oil.

7. The process of claim 6 wherein said repressuring fluid is selected from the group consisting of air, nitrogen, combustion gas, natural gas, methane, water and mixtures thereof.

8. The process of claim 5 wherein said driving agent is selected from the group consisting of air, nitrogen, combustion gas, separator gas, natural gas, methane, water and mixtures thereof.

9. The process of claim 5 wherein said inert gas is selected from the group consisting of methane, natural gas, separator gas, flue gas, nitrogen, air and mixtures thereof. 

2. The process of claim 1 wherein said inert gas is selected from the group consisting of methane, natural gas, separator gas, flue gas, nitrogen, air and mixtures thereof.
 3. The process of claim 2 wherein said inert gas is natural gas.
 4. The process of claim 1 wherein said driving agent is selected from the group consisting of air, nitrogen, combustion gas, separator gas, natural gas, methane, water and mixtures thereof.
 5. A process for recovering oil from an oil-bearing reservoir traversed by at least one injection well and one production well, comprising the steps of: a. injecting into said reservoir via said injection well a slug comprising carbon dioxide and an inert gas, said slug being capable of forming a conditionally miscible transition zone with said oil at the pressure and temperature of said reservoir, said slug being injected in amounts sufficient to establish said transition zone, b. injecting into said reservoir via said injection well a driving fluid and driving said slug and said oil thru said reservoir towards said production well, c. producing said oil via said production well.
 6. The process of claim 5 wherein step (a) is preceded by injection of a fluid to repressure said reservoir to the minimum conditional miscibility pressure at which said slug is miscible with said oil.
 7. The process of claim 6 wherein said repressuring fluid is selected from the group consisting of air, nitrogen, combustion gas, natural gas, methane, water and mixtures thereof.
 8. The process of claim 5 wherein said driving agent is selected from the group consisting of air, nitrogen, combustion gas, separator gas, natural gas, methane, water and mixtures thereof.
 9. The process of claim 5 wherein said inert gas is selected from the group consisting of methane, natural gas, separator gas, flue gas, nitrogen, air and mixtures thereof. 